When an engine starts, the crank shaft of the engine is driven to rotate by rotational force outputting means such as a starter. When the crank shaft rotates, friction in the engine and the compression pressure in, particularly, a cylinder in the compression stroke act as a resistance to the rotation.
If the resistive force to the rotation is too high, the engine may stop running immediately before the top dead center of the cylinder in the compression stroke and fail to start. In a warm environment, in particular, the compression pressure increases significantly, so that the start failure is likely to occur.
To avoid such a start failure, there is a technique of making the rotational force outputting means intermittently apply the torque in the forward direction or alternately reverse the direction of the applied torque when the engine stops running during starting of the engine (see JP03-3969A, for example).
According to the conventional technique, since the torque in the forward direction is intermittently applied, or the direction of the applied torque is alternately reversed, the pressure in the cylinder is released when the torque is cut off, the frictional force changes from static friction to dynamic friction and therefore decreases, and an inertial torque occurs, so that the engine can more easily start.
There is another technique of making the rotational force outputting means run the engine in the reverse direction at the beginning of starting of the engine and then run the engine in the forward direction (see JP07-71350A).
According to this technique, the pressure in the cylinder is released when the torque is cut off, the frictional force changes from static friction to dynamic friction and therefore decreases, and an inertial torque occurs, so that the engine can more easily start.
There is another technique of rotating the crank shaft in the reverse direction to a predetermined position immediately after the engine is stopped to provide for the next engine start (see JP396941B, JP2002-130095A and JP2002-332938A, for example).
According to this technique, the inertial force can be increased to improve the startability of the engine.